build: adjust $(TARGET).efi creation in arch/arm

There is no need to try to guess a relative path to the "xen.efi" file,
we can simply use $@. Also, there's no need to use `notdir`, make
already do that work via $(@F).

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>

xen/arch/arm/Makefile

@@ -95,7 +95,7 @@ endif
 $(TARGET): $(TARGET)-syms
 	$(OBJCOPY) -O binary -S $< $@
 ifeq ($(CONFIG_ARM_64),y)
-	ln -sf $(notdir $@)  ../../$(notdir $@).efi
+	ln -sf $(@F) $@.efi
 endif
 
 ifeq ($(CONFIG_LTO),y)